1. Field of the Invention
The present invention relates to a switching power source circuit and to a motor driving circuit employing the switching power source circuit.
2. Description of the Prior Art
Various switching power source circuits have been proposed in recent years. The switching power source circuits are applicable for driving loads such as motors.
Usually, lubrication of-moving portions of a motor gradually deteriorates to increase friction in the moving portions as an ambient temperature falls, to thereby increase a load on a power source circuit that drives the motor. The power source circuit, therefore, must have a margin for allowing a 30% to 50% increase in the load applied to the circuit at a temperature of about -20 degrees centigrade.
An example of a conventional switching power source circuit will be explained with reference to FIG. 1.
The power source circuit 100 has a line filter 101 for removing noise from alternating current (AC) source power to the circuit 100. The noise-removed AC source power is rectified through a rectifier circuit 103 and smoothed by a capacitor C1. The smoothed direct current (DC) source power is supplied to a control circuit 105 through an activation resistor R1 and to primary windings TR1a and TR1b of a converter transformer (hereinafter referred to as the transformer) TR1.
An output voltage of a secondary winding TR1c of the transformer TR1 is supplied to a voltage limit circuit 107 through rectifying diodes D1 and D3, a choke coil CH1, and a smoothing capacitor C3. The voltage limit circuit 107 detects the output voltage of the secondary winding TR1c of the transformer TR1, and if the detected voltage has reached a limit value, provides a detection signal to the control circuit 105.
A current limit circuit 109 receives a terminal voltage of a resistor R3 to detect an output current of the secondary winding TR1c of the transformer TR1. If the detected current has reached a limit value, the current limit circuit 109 provides a detection signal to the control circuit 105.
A current limit circuit 111 detects a current on the primary side of the transformer TR1. Namely, the current limit circuit 111 receives a terminal voltage of a resistor R5 to detect the current on the primary side of the transformer TR1. If the detected current has reached a limit value, the current limit circuit 111 provides a detection signal to the control circuit 105.
After receiving the detection signal from any one of the voltage limit circuit 107 and current limit circuits 109 and 111, the control circuit 105 turns OFF a transistor Tr1, thereby limiting the output of the transformer TR1.
The control circuit 105 comprises an oscillation circuit 119 and a PWM controller 121. The oscillation circuit 119 has, as oscillation frequency defining elements, a resistor 115 and a capacitor 117.
A signal having an oscillation frequency determined by the resistor 115 and capacitor 117 turns ON and OFF the transistor Tr1 through the PWM controller 121. As a result, the DC source power is periodically switched and converted into high-frequency AC power. In this way, the resistor 115 and capacitor 117 determine the converting frequency of the switching power source circuit.
The detection signals provided by the voltage limit circuit 107 and current limit circuit 109 according to the DC output voltage and current are sent to the PWM controller 121 of the control circuit 105, which widens an ON pulse width of the transistor Tr1 when the output voltage and current drop, and narrows the ON pulse width of the transistor Tr1 when the output voltage and current rise, thereby maintaining a constant output.
The maximum transmission power of the power source circuit depends on the capacity of the transformer TR1. With the same capacity, the transformer TR1 can transmit larger power if the converting frequency is increased. There is a limit, however, on the converting frequency because the transformer TR1 and transistor Tr1 generate more heat as the converting frequency rises.
Generally, the maximum output power of the conventional power source circuit has no temperature dependency. The power source circuit, therefore, is designed based on a most critical operation condition, i.e., an expected maximum ambient temperature (about 60 to 80 degrees centigrade). Accordingly, the power source circuit has a margin when operated at an ordinary ambient temperature (about 20 to 30 degrees centigrade).
Generally, the converting efficiency of the switching power source circuit increases as its output increases, and at the maximum output, the converting efficiency reaches the maximum. This means that the power source circuit having a large maximum output margin usually operates at low converting efficiency.
As explained above, the conventional power source circuit for driving a load such as a motor having movable portions (frictional portions) must be designed to provide a 30% to 50% larger output at a low temperature compared with an output at an ordinary temperature. This increases the size of the transformer TR1 and transistor Tr1, thereby increasing the cost of the power source circuit. In addition, the conventional power source circuit operates at low converting efficiency at the ordinary temperature because an output at the ordinary temperature is designed to be lower than that at the low temperature.